Numerical method to predict the cavitating flows in hydraulic machinery

With the aid of computational fluid dynamics and the rapid increase in computing power, internal flows in the complex domain of a hydraulic turbomachinery can be well predicted, thus facilitating the design of turbines and pumps. The progress of the CFD code performance enables to reduce the design time by coupling CFD codes with optimization tools. However the optimization procedure still employs difficult aspects, like cavitation. Although the numerical modeling of cavitation has received a great deal of attention, it is still not possible to predict such complex unsteady flow. Cavitation is one of the most important aspects that need to be considered while designing centrifugal pumps for example, since it is a major contributor to failure and inefficiency. And the recently method for designing impeller blades are based on a minimal drop in static pressure on the blade surface. This work presents a three dimensional numerical method able to simulate the steady and unsteady cavitating flows applied in case of a centrifugal pump (ns32). The cavitation model used is recently developed and validate [1]. It is based on the vapor volume fraction transport equation including a source term evaluating vaporization and condensation processes depending on the pressure variation within the flow. The Reynolds-averaged Navier-Stokes equations are used for the mixture of liquid and vapor, which is considered as a single fluid. And the ANSYS CFX code is used to compute the corresponding solution. For steady flow, comparison of the numerical and experimental measurement show that the Shear Stress transport (SST) turbulence model can better reproduce the head pump. Flow structures are identified for several impeller blades and volute tongue relative positions. The computed velocity and pressure fields show a radial thrust. Conformed to experimental visualization, a non axisymmetric cavitation appears in the blades near the tongue.